1. Field of the Invention
The present invention relates to a liquid crystal dispensing apparatus, and more particularly, to a liquid crystal dispensing apparatus capable of determining an amount of liquid crystal material to be dispensed onto a substrate and also capable of determining an amount of liquid crystal material remaining within a liquid crystal container.
2. Discussion of the Background Art
As portable electric devices such as mobile phones, personal digital assistants (PDA), notebook computers, etc., continue to be developed, small, light, and power-efficient flat panel display devices such as liquid crystal displays (LCD), plasma display panels (PDP), field emission displays (FED), vacuum fluorescent displays (VFD), etc., have become the subject of intense research. Due to their ability to be mass-produced, ease in driving, and superior image qualities, LCDs are of particular interest.
LCDs display information on a screen using refractive anisotropic properties of liquid crystal material. Referring to FIG. 1, an LCD 1 typically includes a lower substrate (i.e., a driving device array substrate) 5 connected to an upper substrate (i.e., a color filter substrate) 3 via sealant 9. A layer of liquid crystal material 7 separates the lower and upper substrates 5 and 3. A plurality of pixels (not shown) are formed on the lower substrate 5 and driving devices such as thin film transistors (TFTs) are formed on each of the pixels. A color filter layer is formed on the upper substrate 3 allowing the LCD to express color. Further, pixel electrodes and a common electrode are also formed on the lower and upper substrates 5 and 3, respectively. An alignment layer is formed on both the lower and upper substrates 5 and 3 to uniformly align molecules within the layer of liquid crystal material 7. The molecules within the layer of liquid crystal material may be selectively oriented by the driving devices. Accordingly, as the orientation of the molecules within the liquid crystal material is manipulated, the amount of light transmitted through portions of the LCD may be selectively controlled to convey information.
Fabrication processes for LCD devices may be roughly divided into a driving device array fabrication process, where driving devices are formed on the lower substrate 5, a color filter fabrication process, where the color filter is formed on the upper substrate 3,-and a cell fabrication process. These fabrication processes will now be described with reference to FIG. 2.
Referring to FIG. 2, in the driving device array substrate fabrication process (S101), a plurality pixel areas are formed at crossings of a plurality of gate lines and data lines formed on the lower substrate 5 and thin film transistors arranged in each pixel area are connected to gate lines and corresponding ones of data lines. Also, pixel electrodes are connected to each of the thin film transistors to drive the layer of liquid crystal material. Accordingly, the layer of liquid crystal material may be driven in accordance with a signal applied to the thin film transistor.
In the color filter fabrication process (S104), red (R), green (G), and blue (B) color filter layers for producing color and a common electrode are formed on the upper substrate 3.
The alignment layer is formed on both the lower and upper substrates 5 and 3, respectively. After being formed on the substrates, the alignment layer is rubbed to induce molecules within the layer of liquid crystal material to inherit a predetermined pretilt angle and alignment direction between the lower and upper substrates 5 and 3 (S102 and S105). Subsequently, spacers are dispensed over the lower substrate 5 to maintain a uniform cell gap between the upper and lower substrates (S103). The sealant is applied to an outer portion of the upper substrate 3 (S106) and the lower substrate 5 is pressed and attached to the upper substrate 3 (S107).
The lower and upper substrates 5 and 3 are formed from glass substrates having an area larger in size than any individual panel areas. Accordingly, a plurality of corresponding panel areas where driving devices and color filter layers are may be arranged within the attached glass substrates. Thus, in fabricating individual liquid crystal display panels, the attached glass substrates are cut into individual panels (S108). Subsequently, liquid crystal material is injected through a liquid crystal injection opening into the cell gap formed between the two substrates of each individual liquid crystal display panel (S109). After the liquid crystal material is injected, the liquid crystal injection opening is sealed (S109) and each individual liquid crystal display panel is inspected (S110).
To inject the liquid crystal material through the liquid crystal injection opening, a pressure difference between the exterior and the interior of the liquid crystal display panel is induced. FIG. 3 illustrates a device used to inject liquid crystal material into cell gaps of liquid crystal display panels.
Referring to FIG. 3, liquid crystal material 14 is provided in a container 12 arranged within a vacuum chamber 10 that is connected to a vacuum pump (not shown) capable of creating and maintaining a vacuum within the vacuum chamber. A liquid crystal display panel moving device (not shown) is installed within the vacuum chamber 10 and moves separated liquid crystal display panels down from an upper portion of the container 12 toward the surface of the liquid crystal material 14. In what is known as a liquid crystal injection method, the liquid crystal injection opening 16 of each liquid crystal display panel is arranged to contact the liquid crystal material. Subsequently, nitrogen gas (N2) is pumped into the vacuum chamber to increase the pressure therein from the initial vacuum pressure. As the pressure within the vacuum chamber 10 increases, the liquid crystal material 14 contacting the liquid crystal injection opening 16 is extruded (i.e., injected) into the cell gap of the liquid crystal display panel due to the pressure difference between the interior of the liquid crystal display panel and the interior of the vacuum chamber containing the pumped nitrogen gas. After the cell gap is completely filled with liquid crystal material 14, the injection opening 16 is sealed using a sealant.
Injecting liquid crystal material according to the process described above is disadvantageous, however, at least for the following reasons.
First, the amount of time required to completely inject liquid crystal material 14 into the liquid crystal display panel 1 can be excessively long. For example, the cell gap between the driving device array and the color filter substrates is very narrow (e.g., on the order of a few micrometers) and, therefore, only a very small amount of liquid crystal material can be injected into the liquid crystal display panel at any time. Accordingly, injecting liquid crystal material into a typical 15-inch liquid crystal display panel using the injection process described above may take up to about eight hours. Thus, the time required to fabricate LCDs is unduly increased with the use of the liquid crystal injection process.
Second, the amount of liquid crystal material required by the liquid crystal injection method described above is exceedingly large. While only a small amount of liquid crystal is removed from the container 12, a large amount of liquid crystal may become exposed to the atmosphere or to the nitrogen gas. Accordingly, a large amount of liquid crystal material reacts with, and can be contaminated by, nitrogen or other gases within the atmosphere. As a result, the cost of fabricating LCDs increases because liquid crystal material not injected into the liquid crystal display panel must be discarded after the injection process.